Generally, an optical fiber is manufactured by coating a preform, fabricated by IVPO (Inside Vapor Phase Oxidization) or OVD (Outside Vapor Deposition), with a heating furnace to make its end into a melted state, and then cooling, coating and hardening the preform with drawing the melted portion to a very small diameter.
FIG. 1 shows an optical fiber manufactured as mentioned above. Referring to FIG. 1, the optical fiber includes a core layer 1 for propagating optical signals, a clad layer 2 surrounding the core layer 1 and having a relatively lower refractive index than the core layer 1, and a coating layer 3.
The coating layer 3 protects the optical fiber composed of the core layer 1 and the clad layer 2 from external impacts, and it is formed by coating a coating agent on the surface of the optical fiber being drawn.
However, if a drawing speed of the optical fiber is increased to reduce manufacture costs of the optical fiber, an external air is introduced into the coating unit along the surface of the optical fiber during the optical fiber coating process, and the air does not come out but remains therein as a bubble, which causes deteriorated quality and disconnection of the optical fiber. As the drawing speed is increased, thickness of a boundary layer of the external air introduced into the coating agent along the surface of the optical fiber is increased on the surface of the optical fiber. In this case, stable meniscus form generated when the optical fiber comes in contact with the coating layer is broken, resulting such inferior quality and disconnection.
FIG. 2 shows a conventional coating device, directed to solving the above problem. Referring to FIG. 2, the conventional coating device forms a coating layer by coating a coating agent, introduced through a coating agent inlet 7 formed in one internal side of the coating device, onto the surface of an optical fiber 4 introduced through an introduction passage 5. During such a coating process, before the coating agent is coated on the surface of the optical fiber 4, the coating device sprays gas, which has lower kinetic viscosity than the air introduced into the coating device through a gas inlet 8, on the surface of the optical fiber 4 through nozzles 6 provided to both sides of the introduction passage 5. Since the gas is sprayed, thickness of a boundary layer formed on the optical fiber surface by an external air is decreased. In addition, since the boundary layer is formed on the surface of the optical fiber 4 by the gas having smaller kinetic viscosity than air, the thickness of the boundary layer of the external air that causes generation of bubbles may be reduced though a drawing speed of the optical fiber is increased.
However, since the conventional coating device sprays gas directly to the surface of the optical fiber, the optical fiber being introduced into the coating device is finely vibrated. Accordingly, it may cause change of diameter and deteriorated coating quality of the optical fiber being drawn. This problem is worse in a process circumstance where pressure and amount of gas should be increased as a drawing speed of the optical fiber is increased.